This invention relates to a data signal processing method and apparatus and, more particularly, to such method and apparatus which are useful for functions such as signal bit code conversion and pre-equalization.
It is well known in the art to convert between digital signal codes employing different combinations of binary ONE and ZERO signal bits to represent the same information. One example, employing a magnetic core matrix, is the U.S. Pat. No. 3,011,165 to A. M. Angel et al. However, in these systems, individual bit signals are usually represented in the same way in both codes; that is, a binary ONE bit is represented in the same way in both codes, and a binary ZERO is represented in the same way in both codes.
Binary signal bits can be represented in different ways in both baseband and modulated formats. For example, in baseband, they may be represented in a pulse/no-pulse format, or a bipolar nonreturn-to-zero (NRZ) format, or a biphase Manchester format, or other formats. Similarly, data bit signals in modulated arrangements may be represented in frequency or phase-shift keyed arrangements, amplitude modulation, or frequency modulation. Regardless of the bit representation format, it is sometimes useful to change the individual bit representation format, sometimes also called bit representation code or modulation. In the U.S. Pat. No. 4,100,541 to H. A. Quesnell, Jr., binary signal bits in an NRZ-type of format are converted to the Manchester format by a multiplexing technique. A D. E. Curtis U.S. Pat. No. 3,774,178 employs a two-bit shift register and associated logic clocked at the bit rate to convert NRZ code to a Pouliart code with state transitions in mid-bit of ONEs, and between adjacent ZEROs. "Digital Generation of Data Modulated Waveforms," by S. M. Bozic et al. in Microelectron. Reliab., Vol. 22, No. 4, pp. 759-767, 1982, shows the clocking of each input data bit at a higher rate than the data to select one or the other of two forms of the same input bit state for application to a digital transversal filter which produces a modulator output.
It is also known to store in memory as sample amplitude value sets certain signal wave segments which are themselves of a type usually generated by analog circuits and so can be easily converted to sets of digitally encoded samples for convenient storage. Those segments are then read from addresses which are selectable in response to address signals generated as a joint function of input data bits and some other information dependent function. Segments read out are are combined with another function, such as readout from another memory address, to obtain a desired result which can be restored to analog sample form for use. One example is a phase shift modulator in an M. Choquet et al. U.S. Pat. No. 3,747,024 in which input data bits are converted to a combinational address by circuitry which combines them with a prior address. The stored signal element contents at the combinational address are summed in other circuitry with the contents of a plurality of previous combinational addresses to produce a data output. Similarly in a Glasson et al. U.S. Pat. No. 3,935,386, a phase modulated carrier is synthesized by addressing a stored carrier segment with an address generated by circuitry which combines an applied data element and the phase of a previously selected segment. Successive segments so addressed are overlapped and blended in additional circuitry to form the synthesized wave. More recently, a T. M. Burke et al. U.S. Pat. No. 4,410,955 shows a data stream shaping method in which extensive logic circuitry detects information state transitions in received data, selects a corresponding sinusoidal segment from a register for output, and by additional circuitry fills between transitions with an appropriate logic level signal.
The extensive circuitry required, in addition to the memory facility, in the foregoing memory-based wave generating arrangements occupy significant semiconductor space in integrated circuit embodiments. Also, the associated signal combining functions necessarily performed in that circuitry tend to limit the flexibility of practical application in terms of, e.g., applying predistortion, frequency band limiting, signal multiplexing, or types of input signals that can be accommodated.
In preparing a converted bit code for, e.g., radio transmission, certain undesired effects sometimes occur. Thus, in the Manchester bit encoder of FIG. 10 in "Advanced Mobile Phone Service--A Subscriber Set for the Equipment Test," by R. E. Fisher, Bell System Technical Journal, January 1979, pages 123-143, a Bessel low-pass filter is employed, following an exclusive ORing operation and integration. The filter maintains the phase relationships among the different components of the data signal and band-limits the signal to reduce the amplitude of frequency components thereof outside of the allotted radio frequency channel. However, such a filter has an output amplitude that is data-pattern sensitive. Such pattern sensitivity in output amplitude has been found to impact adversely the system error rate, and it also makes that error rate pattern-sensitive.